MgSiO3 Glass Research Articles

X-ray Raman scattering study of MgSiO3 glass at high pressure: implication for triclustered MgSiO3 melt in Earth's mantle.

Silicate melts at the top of the transition zone and the core-mantle boundary have significant influences on the dynamics and properties of Earth's interior. MgSiO3-rich silicate melts were among the primary components of the magma ocean and thus played essential roles in the chemical differentiation of the early Earth. Diverse macroscopic properties of silicate melts in Earth's interior, such as density, viscosity, and crystal-melt partitioning, depend on their electronic and short-range local structures at high pressures and temperatures. Despite essential roles of silicate melts in many geophysical and geodynamic problems, little is known about their nature under the conditions of Earth's interior, including the densification mechanisms and the atomistic origins of the macroscopic properties at high pressures. Here, we have probed local electronic structures of MgSiO3 glass (as a precursor to Mg-silicate melts), using high-pressure x-ray Raman spectroscopy up to 39 GPa, in which high-pressure oxygen K-edge features suggest the formation of tricluster oxygens (oxygen coordinated with three Si frameworks; 3O) between 12 and 20 GPa. Our results indicate that the densification in MgSiO3 melt is thus likely to be accompanied with the formation of triculster, in addition to a reduction in nonbridging oxygens. The pressure-induced increase in the fraction of oxygen triclusters >20 GPa would result in enhanced density, viscosity, and crystal-melt partitioning, and reduced element diffusivity in the MgSiO3 melt toward deeper part of the Earth's lower mantle.

Local Structure of Magnesium in Silicate Glasses: A 25Mg 3QMAS NMR Study

We have reported the 25Mg triple-quantum magic-angle spinning (3QMAS) NMR spectra of silicate glasses. The two-dimensional spectra suggest that the magnesium ions in MgSiO3, CaMgSi2O6, Ca2MgSi2O7, Mg3Al2Si3O12, and Li2MgSi2O6 glasses are mainly in octahedral environments, although in Na2MgSi2O6, K2MgSi2O6, and K2MgSi5O12 glasses they form tetrahedral species. We discussed the coordination environments of Mg based on the field strength of competing Mg2+, Ca2+, Na+, K+, and Li+ cations, and convincingly demonstrated that there is a correlation between them. These results indicate that the two-dimensional NMR spectroscopy such as MQMAS technique is a very useful method to analyze the local environments of nonframework cations in noncrystalline solids.

Structural change of MgSiO3 glass by static compression

Structural change of MgSiO3 glass by static compression

Molecular dynamics study of CaSiO3–MgSiO3 glasses under high pressure

The pressure-induced structural evolutions ofCaSiO3–MgSiO3 glasses have been examined by means of molecular dynamics simulation. Ourcalculations revealed that Si coordination remained unchanged up to 15 GPa, whilemodifier cations caused significant changes in the short-range order structure.In the present study, we conclude that the main compression mechanisms forCaSiO3–MgSiO3 glasses are: (1) the Si–O–Si angle reduction, (2) the coordination increase of Ca and Mgcations, and (3) the compaction in the medium-range scale. Furthermore, small changes in theQn distribution suggest pressure-induced disproportionation reactions. Similar pressure responses betweenCaSiO3–MgSiO3 glasses may imply that the structural changes ofSiO4 framework units are more significant than those of interstitial cations, Ca and Mg.

Shock‐induced melting of MgSiO3 perovskite and implications for melts in Earth's lowermost mantle

New shock wave equation of state (EOS) data for enstatite and MgSiO3 glass constrain the density change upon melting of Mg‐silicate perovskite up to 200 GPa. The melt becomes denser than perovskite near the base of Earth's lower mantle. This inference is confirmed by shock temperature data suggesting a negative pressure‐temperature slope along the melting curve at high pressure. Although melting of Earth's mantle involves multiple phases and chemical components, this implies that the partial melts invoked to explain anomalous seismic velocities in the lowermost mantle may be dynamically stable.

Shock‐compressed MgSiO3 glass, enstatite, olivine, and quartz: Optical emission, temperatures, and melting

Optical emission of MgSiO3 glass, enstatite, olivine, and quartz under shock wave compression was investigated with optical pyrometry at discrete wavelengths ranging from visible to near infrared. We develop a new analysis of optical emission that does not require a gray body assumption. Instead, at each wavelength, the optical linear absorption coefficients (a) and blackbody spectral radiances (Lλb) of shocked and unshocked materials were obtained by nonlinear fitting to the time‐resolved radiance from the target assembly. The absorption spectra of unshocked samples corresponding to the measured values of a reproduce those from independent static optical spectroscopic measurements. The measured values of a (ranging from 7 to 56 mm−1) for shocked samples indicate that shock‐induced high‐pressure phases (including melt) can be regarded essentially as black bodies in the optical range investigated, although starting phases such as enstatite and olivine have band‐like spectra at ambient conditions. The effect of emission from the air gap at the driver sample interface on the recorded radiance can be resolved, but a and Lλb cannot be separated for this component of the signal. The shock velocity‐particle velocity relationships of these silicates derived from radiance history are in accord with previous investigations using independent techniques. Given the limited amount of shock wave data, possible high‐pressure melting curves of Mg‐perovskite and its assemblage with periclase are deduced; their melting temperatures near the core‐mantle boundary (CMB) being 6000 ± 500 K and 4000 ± 300 K, respectively. It is proposed that Mg‐perovskite melts with density increase at the CMB pressure.

Low‐temperature crystallization of MgSiO3 glasses under electron irradiation: Possible implications for silicate dust evolution in circumstellar environments

Abstract— Synthetic MgSiO3 glasses were irradiated at room temperature by 300 keV electrons in a transmission electron microscope (TEM). One of the samples had been previously irradiated by 50 keV He+ ions. Electron irradiation induces the nucleation and growth of randomly oriented nanometer‐sized crystallites. The crystallites first consist of MgO and subsequently of forsterite (Mg2SiO4). Both are seen to form within an amorphous SiO2 matrix. The rate of crystallization of the samples has been monitored by conventional TEM imaging and electron diffraction. The sample that had been pre‐irradiated with He+ ions is found to transform faster than the as‐quenched glass. The crystallization of metastable MgSiO3 glasses is explained by ionizing radiation‐induced elemental diffusion that allows the reorganization of matter into a more favourable thermodynamic state. These results show that ionizing radiation interactions could account for crystal formation as observed in infrared spectroscopy in some young stellar environments.

Elastic properties of silicate melts up to 2350 K from Brillouin scattering

Compressional wave velocities (Vp) have been determined up to 2350 K on CaSiO3, MgSiO3, CaMgSi2O6, CaAl2Si2O8 and Ca3Al2Si3O12 glasses and melts from Brillouin‐scattering experiments made with a 180° backscattering geometry. At the glass transition, the decrease of Vp with increasing temperatures becomes much stronger and the width of the Brillouin lines begins to increase markedly. At the highest temperatures investigated, Vp is similar to the relaxed values determined in acoustic measurements for several of these melts. This indicates that the configurational degrees of freedom of these liquids have become accessible within the very short timescale of Brillouin scattering experiments: equilibrium compressibilities of silicate melts can thus be determined with this technique. Our measurements also suggest that the shear modulus at infinite frequency of silicate melts could vary with either temperature or composition more strongly than assumed currently.

The structure analyses of pyrope (Mg3Al2Si3Ol2) and grossular (Ca3Al2Si3O12) glasses by X-ray diffraction method.

The structures of Mg3Al2Si3O12 and Ca3Al2Si3O12 glasses were investigated by means of the radial distribution function (RDF) analysis based on X-ray diffraction data. The Al and Si atoms in these glasses are essentially in four-fold coordination. The T–O (T=A1 and Si) distance of Mg3Al2Si3O12 glass (1.68A) estimated from the peak position of radial distribution function curve is almost equal to that of Ca3Al2Si3O12 glass (1.70A). These T–O distances are consistent with the common average T–O distance in AlO4 and SiO4 tetrahedra. Mg–O distance in Mg3Al2Si3O12 glass is 2.1A and coordination number of Mg atom is 4.8. These values are smaller than those in pyroxene and pyrope crystals. Ca–O distances in Ca3Al2Si3O12 glass is 2.4A and coordination number of Ca atom is 7.1–7.4. These values are in conformity with those obtained in grossular and diopside crystals. The short range structures of Mg3Al2Si3O12 and Ca3Al2Si3O12 glasses, the oxygen coordination around T, Mg, and Ca atoms resemble those of MgSiO3 and CaSiO3 glasses, respectively. Especially, the G(r) curve of Mg3Al2Si3O12 glass is very similar to that of MgSiO3 glass.

Molecular dynamics simulations of pressure and temperature effects on MgSiO3 and Mg2SiO4 melts and glasses

Ab-initio interionic potentials for Mg2+, Si4+, and O2− have been used in molecular dynamics (MD) simulations to investigate diffusivity changes, pressure-induced structural transitions, and temperature effects on polymerization in MgSiO3 and Mg2SiO4 melts and glasses. The potential gives reasonable agreement with the 0.1 MPa radial distribution function of MgSiO3 glass. Maxima in the diffusion coefficients of Si4+ and O2− occur as pressure is increased on the MgSiO3 melt. The controlling structural mechanism for this behavior is the Q1 species of SiO4 tetrahedra. Mg2+ diffusion coefficients decrease monotonically with pressure in both melt compositions. Increasing Mg2+ coordination number and population of 3- and 4-membered SiO4 rings with pressure combine to hinder translation of the Mg2+ ions. The dominant changes in structure with pressure are a decrease in the intertetrahedral (Si-O--Si) angle up to approximately 4 g/cm3 and coordination changes of the ions above this density. Temperature effects on viscosity in these simulated melts are indirectly studied by analyzing polymerization changes with temperature. Polymerization and coordination numbers increase with decreasing temperature and a small quench rate effect is observed. Fair agreement is found between the MD simulations and experimental equation of state for Mg2SiO4, but the equation of state predictions for MgSiO3 melts are much less accurate. The zero pressure volume, V 0, is significantly higher and K 0 is lower in the simulations than empirical values. The inadequacies reflect error in using the ionic approximation for polymerized systems and a need to collect more data for a variety of molecular configurations in the development of ab-initio potentials.

Instantaneous structure of an MgSiO3 melt simulated by molecular dynamics

A series of molecular dynamics (MD) calculations1 has been carried out to visualize the atomic configuration in MgSiO3 melt and glass. The ‘computer-generated’ configuration seemed to be realistic. At zero pressure the Si–O–Si angle distribution gave a clear peak at ∼145° in agreement with the result of ordinary X-ray study on vitreous SiO2 (ref. 2). Coordination of O around Mg was irregular, because the whole structure is dominated by packing of the rather regular SiO4 tetrahedra which are linked together by sharing O atoms at corners. The ‘melt’ also showed a partial conversion from a 4- to 6-coordination state on ‘compression’ within a reasonable density range, accompanied by a steady decrease in the Si–O–Si angles. The MD calculation, which is simply the numerical integration of the classical newtonian equations of motion for many particles in a hypothetical periodic space, has been successfully applied to various molten ionic salt systems1. Following Woodcock et al's3 first application of MD to vitreous SiO2, we decided to extend the MD calculation to systems of geochemical importance and report our results here.